Strategy for Manipulation of Cheese Flora Using Combinations of Lacticin 3147-Producing and -Resistant Cultures

The aim of the present study was to develop adjunct strains which can grow in the presence of bacteriocin produced by lacticin 3147-producing starters in fermented products such as cheese. A Lactobacillus paracasei subsp. paracasei strain (DPC5336) was isolated from a well-flavored, commercial cheddar cheese and exposed to increasing concentrations (up to 4,100 arbitrary units [AU]/ml) of lantibiotic lacticin 3147. This approach generated a stable, more-resistant variant of the isolate (DPC5337), which was 32 times less sensitive to lacticin 3147 than DPC5336. The performance of DPC5336 was compared to that of DPC5337 as adjunct cultures in two separate trials using either Lactococcus lactis DPC3147 (a natural producer) or L. lactis DPC4275 (a lacticin 3147-producing transconjugant) as the starter. These lacticin 3147-producing starters were previously shown to control adventitious nonstarter lactic acid bacteria in cheddar cheese. Lacticin 3147 was produced and remained stable during ripening, with levels of either 1,280 or 640 AU/g detected after 6 months of ripening. The more-resistant adjunct culture survived and grew in the presence of the bacteriocin in each trial, reaching levels of 10⁷ CFU/g during ripening, in contrast to the sensitive strain, which was present at levels 100- to 1,000-fold lower. Furthermore, randomly amplified polymorphic DNA-PCR was employed to demonstrate that the resistant adjunct strain comprised the dominant microflora in the test cheeses during ripening.     

Assessment of the antifungal activity of <Emphasis Type="Italic">Lactobacillus</Emphasis> and <Emphasis Type="Italic">Pediococcus</Emphasis> spp. for use as bioprotective cultures in dairy products

Fungi are commonly involved in dairy product spoilage and the use of bioprotective cultures can be a complementary approach to reduce food waste and economic losses. In this study, the antifungal activity of 89 Lactobacillus and 23 Pediococcus spp. isolates against three spoilage species, e.g., Yarrowia lipolytica, Rhodotorula mucilaginosa and Penicillium brevicompactum, was first evaluated in milk agar. None of the tested pediococci showed antifungal activity while 3, 23 and 43 lactobacilli isolates showed strong antifungal activity or total inhibition against Y. lipolytica, R. mucilaginosa and P. brevicompactum, respectively. Then, the three most promising strains, Lactobacillus paracasei SYR90, Lactobacillus plantarum O_VI9 and Lactobacillus rhamnosus BIO_III28 at initial concentrations of 10⁵ and 10⁷ CFU/ml were tested as bioprotective cultures against the same fungal targets in a yogurt model during a 5-week storage period at 10 °C. While limited effects were observed at 10⁵ CFU/ml inoculum level, L. paracasei SYR90 and L. rhamnosus BIO_III28 at 10⁷ CFU/ml respectively retarded the growth of R. mucilaginosa and P. brevicompactum as compared to a control without selected cultures. In contrast, growth of Y. lipolytica was only slightly affected. In conclusion, these selected strains may be good candidates for bioprotection of fermented dairy products.     

Urea Hydrogen Peroxide Reduces the Numbers of Lactobacilli, Nourishes Yeast, and Leaves No Residues in the Ethanol Fermentation

Urea hydrogen peroxide (UHP) at a concentration of 30 to 32 mmol/liter reduced the numbers of five Lactobacillus spp. (Lactobacillus plantarum, L. paracasei, Lactobacillus sp. strain 3, L. rhamnosus, and L. fermentum) from ~10⁷ to ~10² CFU/ml in a 2-h preincubation at 30°C of normal-gravity wheat mash at ~21 g of dissolved solids per ml containing normal levels of suspended grain particles. Fermentation was completed 36 h after inoculation of Saccharomyces cerevisiae in the presence of UHP, even when wheat mash was deliberately contaminated (infected) with L. paracasei at ~10⁷ CFU/ml. There were no significant differences in the maximum ethanol produced between treatments when urea hydrogen peroxide was used to kill the bacteria and controls (in which no bacteria were added). However, the presence of L. paracasei at ~10⁷ CFU/ml without added agent resulted in a 5.84% reduction in the maximum ethanol produced compared to the control. The bactericidal activity of UHP is greatly affected by the presence of particulate matter. In fact, only 2 mmol of urea hydrogen peroxide per liter was required for disinfection when mashes had little or no particulate matter present. No significant differences were observed in the decomposition of hydrogen peroxide in normal-gravity wheat mash at 30°C whether the bactericidal agent was added as H₂O₂ or as urea hydrogen peroxide. NADH peroxidase activity (involved in degrading H₂O₂) increased significantly (P = 0.05) in the presence of 0.75 mM hydrogen peroxide (sublethal level) in all five strains of lactobacilli tested but did not persist in cells regrown in the absence of H₂O₂. H₂O₂-resistant mutants were not expected or found when lethal levels of H₂O₂ or UHP were used. Contaminating lactobacilli can be effectively managed by UHP, a compound which when used at ca. 30 mmol/liter happens to provide near-optimum levels of assimilable nitrogen and oxygen that aid in vigorous fermentation performance by yeast.     

Comparative Survival Rates of Human-Derived Probiotic Lactobacillus paracasei and L. salivarius Strains during Heat Treatment and Spray Drying

Spray drying of skim milk was evaluated as a means of preserving Lactobacillus paracasei NFBC 338 and Lactobacillus salivarius UCC 118, which are human-derived strains with probiotic potential. Our initial experiments revealed that NFBC 338 is considerably more heat resistant in 20% (wt/vol) skim milk than UCC 118 is; the comparable decimal reduction times were 11.1 and 1.1 min, respectively, at 59°C. An air outlet temperature of 80 to 85°C was optimal for spray drying; these conditions resulted in powders with moisture contents of 4.1 to 4.2% and viable counts of 3.2 × 10⁹ CFU/g for NFBC 338 and 5.2 × 10⁷ CFU/g for UCC 118. Thus, L. paracasei NFBC 338 survived better than L. salivarius UCC 118 during spray drying; similar results were obtained when we used confocal scanning laser microscopy and LIVE/DEAD BacLight viability staining. In addition, confocal scanning laser microscopy revealed that the probiotic lactobacilli were located primarily in the powder particles. Although both spray-dried cultures appeared to be stressed, as shown by increased sensitivity to NaCl, bacteriocin production by UCC 118 was not affected by the process, nor was the activity of the bacteriocin peptide. The level of survival of NFBC 338 remained constant at ~1 × 10⁹ CFU/g during 2 months of powder storage at 4°C, while a decline in the level of survival of approximately 1 log (from 7.2 × 10⁷ to 9.5 × 10⁶ CFU/g) was observed for UCC 118 stored under the same conditions. However, survival of both Lactobacillus strains during powder storage was inversely related to the storage temperature. Our data demonstrate that spray drying may be a cost-effective way to produce large quantities of some probiotic cultures.     

Identification of lactobacilli isolated in traditional ripe wheat sourdoughs by using molecular methods

Eleven sourdoughs from Molise region (Southern-Italy) were subjected to microbiological analyses in order to select predominant lactobacilli species to be utilised as starter culture for bread production. A multiple approach was used, consisting of the growth in different culture media, DGGE analysis, 16S rRNA gene sequencing and RAPD-PCR typing. Forty-three lactobacilli were identified and four different species, facultatively or obligately heterofermentative lactobacilli, were found. Lactobacillus plantarum and Lactobacillus brevis represented the prevailing lactobacilli, while Lactobacillus casei and Lactobacillus paracasei ssp. paracasei were detected only in few samples. The use of different media demonstrated that there is no efficient medium for the study of sourdoughs and the cultivation in different substrates remains the best tool to obtain a picture of lactic acid bacteria population. DGGE and 16S rRNA gene sequencing allowed to obtain a reliable identification of strains, while RAPD-PCR resulted a suitable method for typing lactobacilli at strain level.     

Fermentation of a milk-soymilk and Lycium chinense Miller mixture using a new isolate of Lactobacillus paracasei subsp. paracasei NTU101 and Bifidobacterium longum

A milk–soymilk mixture was fermented using Lactobacillus paracasei subsp. paracasei NTU101 and Bifidobacterium longum BCRC11847 at different inoculum ratios (1:1, 1:2, 1:5, 2:1, and 5:1). When the inoculum ratio was 1:2, the cell numbers of both strains were balanced after 12 h of cultivation. The pH and titratable acidity were very similar at the various inoculum ratios of cultivation. The milk–soymilk mixture was supplemented with 5, 10, 15, and 20% Lycium chinense Miller juice and fermented with Lactobacillus paracasei subsp. paracasei NTU101 and B. longum BCRC11847. Sensory evaluation results showed that supplementation with 5% Lycium chinense Miller juice improved the acceptability of the fermented milk–soymilk. The fermented beverage was stored at 4°C for 14 days; variations in pH and titratable acidity were slight. The cell numbers of L. paracasei subsp. paracasei NTU101 and B. longum BCRC11847 in the fermented beverage were maintained at 1.2×10⁹ CFU/ml and 6.3×10⁸ CFU/ml, respectively, after 14 days of storage.     

Inhibition of Listeria innocua in Cheddar Cheese by Addition of Nisin Z in Liposomes or by In Situ Production in Mixed Culture

The effect of addition of purified nisin Z in liposomes to cheese milk and of in situ production of nisin Z by Lactococcus lactis subsp. lactis biovar diacetylactis UL719 in the mixed starter on the inhibition of Listeria innocua in cheddar cheese was evaluated during 6 months of ripening. A cheese mixed starter culture containing Lactococcus lactis subsp. lactis biovar diacetylactis UL719 was selected for high-level nisin Z and acid production. Experimental cheddar cheeses were produced on a pilot scale, using the selected starter culture, from milk with added L. innocua (10⁵ to 10⁶ CFU/ml). Liposomes with purified nisin Z were prepared from proliposome H and added to cheese milk prior to renneting to give a final concentration of 300 IU/g of cheese. The nisin Z-producing strain and nisin Z-containing liposomes did not significantly affect cheese production and gross chemical composition of the cheeses. Immediately after cheese production, 3- and 1.5-log-unit reductions in viable counts of L. innocua were obtained in cheeses with encapsulated nisin and the nisinogenic starter, respectively. After 6 months, cheeses made with encapsulated nisin contained less than 10 CFU of L. innocua per g and 90% of the initial nisin activity, compared with 10⁴ CFU/g and only 12% of initial activity in cheeses made with the nisinogenic starter. This study showed that encapsulation of nisin Z in liposomes can provide a powerful tool to improve nisin stability and inhibitory action in the cheese matrix while protecting the cheese starter from the detrimental action of nisin during cheese production.     

Characterization of dominant microbiota of a Ghanaian fermented milk product, nyarmie, by culture- and nonculture-based methods

AIMS: To characterize the predominant micro-organisms in a Ghanaian traditional fermented dairy product, nyarmie, made from cows' milk, using both culture- and nonculture-based methods. METHODS AND RESULTS: Samples of nyarmie were analysed from three production sites in Accra, by determining the counts on selective culture media. The microbial diversity occurring in nyarmie was also evaluated by 16S/18S ribosomal DNA PCR amplification and denaturing gradient gel electrophoresis. Results showed that nyarmie contained lactococci and lactobacilli in the range of 10(8) and 10(10) CFU ml(-1), respectively, and yeasts at around 10(7) CFU ml(-1). The pH ranged between 3.49 and 4.25. The predominant lactic acid bacteria (LAB) in nyarmie were Leuconostocmesenteroides ssp. mesenteroides, Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lact.helveticus, Lact. delbrueckii ssp. lactis and Lactococcus lactis, while Saccharomyces cerevisiae was the predominant yeast species. Lactobacillus delbrueckii ssp. delbrueckii was not detected by cultivation but its predominance was revealed by PCR-DGGE analysis. CONCLUSIONS: The flora in products from different producers varied in the LAB composition present and may result in variations in product quality. SIGNIFICANCE AND IMPACT OF THE STUDY: Development and use of starter cultures for nyarmie may be beneficial in improving the consistency of product quality.     

Identification of interacting mixed cultures of lactic acid bacteria by their exclusion from a model predicting the acidifying activity of non-interacting mixed cultures

A model predicting the acidifying activity of mixed cultures of lactic acid bacteria and based on the lack of interaction between the strains has been investigated to identify interacting cultures. Three mixed cultures with Streptococcus thermophilus TH3 and ST7 and Lactobacillus delbrueckii ssp. bulgaricus LB10 were grown on milk. The acidifying activities of the two mixed cultures TH3/LB10 and TH3/ST7 were predicted accurately by the model, with mean prediction errors of 7.7% and 14.1%, respectively. However, the model underestimated the acidifying activity of the mixed culture ST7/LB10, with a mean prediction error of 43.5%, which provides evidence of positive interaction between the strains ST7 and LB10 during acidification. Received: 26 April 1999 / Received revision: 16 June 2000 / Accepted: 18 June 2000     

Development of a Probiotic Cheddar Cheese Containing Human-Derived Lactobacillus paracasei Strains

Cheddar cheese was manufactured with either Lactobacillus salivarius NFBC 310, NFBC 321, or NFBC 348 or L. paracasei NFBC 338 or NFBC 364 as the dairy starter adjunct. These five strains had previously been isolated from the human small intestine and have been characterized extensively with respect to their probiotic potential. Enumeration of these strains in mature Cheddar cheese, however, was complicated by the presence of high numbers (>10⁷ CFU/g of cheese) of nonstarter lactic acid bacteria, principally composed of lactobacilli which proliferate as the cheese ripens. Attempts to differentiate the adjunct lactobacilli from the nonstarter lactobacilli based on bile tolerance and growth temperature were unsuccessful. In contrast, the randomly amplified polymorphic DNA method allowed the generation of discrete DNA fingerprints for each strain which were clearly distinguishable from those generated from the natural flora of the cheeses. Using this approach, it was found that both L. paracasei strains grew and sustained high viability in cheese during ripening, while each of the L. salivarius species declined over the ripening period. These data demonstrate that Cheddar cheese can be an effective vehicle for delivery of some probiotic organisms to the consumer.     

Elimination of Escherichia coli O157:H7 from fermented dry sausages at an organoleptically acceptable level of microencapsulated allyl isothiocyanate

Four sausage batters (17.59% beef, 60.67% pork, and 17.59% pork fat) were inoculated with two commercial starter culture organisms (>7 log(10) CFU/g Pediococcus pentosaceus and 6 log(10) CFU/g Staphylococcus carnosus) and a five-strain cocktail of nonpathogenic variants of Escherichia coli O157:H7 to yield 6 to 7 log(10) CFU/g. Microencapsulated allyl isothiocyanate (AIT) was added to three batters at 500, 750, or 1,000 ppm to determine its antimicrobial effects. For sensory analysis, separate batches with starter cultures and 0, 500, or 750 ppm microencapsulated AIT were produced. Sausages were fermented at < or =26 degrees C and 88% relative humidity (RH) for 72 h. Subsequently sausages were dried at 75% RH and 13 degrees C for at least 25 days. The water activity (a(w)), pH, and levels of starter cultures, E. coli O157:H7, and total bacteria were monitored during fermentation and drying. All sausages showed changes in the initial pH from 5.57 to 4.89 and in a(w) from 0.96 to 0.89 by the end of fermentation and drying, respectively. Starter culture numbers were reduced during sausage maturation, but there was no effect of AIT on meat pH reduction. E. coli O157:H7 was reduced by 6.5 log(10) CFU/g in sausages containing 750 and 1,000 ppm AIT after 21 and 16 days of processing, respectively. E. coli O157:H7 numbers were reduced by 4.75 log(10) CFU/g after 28 days of processing in treatments with 500 ppm AIT, and the organism was not recovered from this treatment beyond 40 days. During sensory evaluation, sausages containing 500 ppm AIT were considered acceptable although slightly spicy by panelists.     

Selenite-stress selected mutant strains of probiotic bacteria for Se source production

Selenium deficiency is a major health problem worldwide for about 1 billion people. Bacterial cells usually possess low tolerance to selenite stress and also low ability to reduce high concentrations of toxic selenite. Here, high tolerance to selenite and selenium bioaccumulation capability were developed in mutated clones of probiotic and starter bacteria including Enterococcus faecium, Bifidobacterium animalis ssp. lactis, Lactobacillus casei and Lactococcus lactis ssp. lactis by food-level strain development process and clone selection. All mutant clones possessed increased glutathione concentration and glutathione reductase activity. The selenite treatment increased further these values in L. casei mutant strain pointing at a different selenite reduction pathway and/or stress response in this organism. Considerable conversion of selenite to cell bound selenium forms with a concomitant high biomass production was detected in E. faecium and B. animalis ssp. lactis cultures. Possible application of these strains as food and feed supplements is under investigation.     

Safety evaluation of Lactobacillus paracasei subsp. paracasei LC-01, a probiotic bacterium

The safety of Lactobacillus paracasei subsp. paracasei LC-01 was evaluated for its use as a potential probiotic. In our in vitro study, the antibiotic resistance and the ability to produce biogenic amine were determined. The results showed that the strain was sensitive to all tested antibiotics and did not produce biogenic amine except for tyramine. The oral toxicity of this strain was evaluated in Balb/C mice. One hundred mice were divided into 10 groups. Four groups were administered 0, 10⁸, 10⁹, or 10¹⁰ CFU/mouse per day dissolved in saline solution respectively, for 28 days. Three groups were injected intraperitoneally with 10⁹ CFU/mouse dissolved in saline solution, and were killed 2, 5, and 10 days after injection. The last 3 groups were injected with the vehicle as controls respectively. The results showed that oral administration of the strain had no adverse effects on mouse body weight and that there was no treatment-associated bacterial translocation. Intraperitoneal administration caused a significant translocation to liver, spleen and kidney. However, this translocation did not cause illness or death throughout the experiment. The results suggest that L. paracasei subsp. paracasei LC-01 is likely to be safe for human consumption.     

Changes in steady state on introduction of a Lactobacillus contaminant to a continuous culture ethanol fermentation

Lactobacillus paracasei was introduced as a contaminant into a multistage continuous culture ethanol fermentation system at ratios of 1:100, 1:1, and 70:1 with Saccharomyces cerevisiae, but failed to overtake the yeast. None of the inoculation ratios allowed L. paracasei to affect S. cerevisiae in the first fermentor in the multistage system. S. cerevisiae remained constant at ∼3×10⁷ CFU/ml regardless of the bacterial inoculation level, and even at the 70:1 inoculation ratio, glucose, ethanol, and lactic acid concentrations did not change from the steady-state concentrations seen before bacterial inoculation. However, L. paracasei decreased steadily from its initial inoculation level of ∼2.2×10⁹ CFU/ml and stabilized at 3.7×10⁵ CFU/ml after 10 days of steady-state operation. Both organisms then persisted in the multistage system at an approximate L. paracasei/S. cerevisiae ratio of 1:100 which confirms that, in continuous fuel ethanol production, it would be difficult to eliminate this bacterium. Only when the pH was controlled at 6.0 in fermentor 1 (F1) were changes seen which would affect the multistage system. Ethanol concentration then decreased by 44% after 4 days of pH-controlled operation. This coincided with an increase in L. paracasei to >10¹⁰ CFU/ml, and a 4× increase in lactic acid concentration to 20 g/l. When the clarified contents from other fermentors (F2–F5) in the multistage system were used as growth media, L. paracasei was not able to grow in batch culture. This indicated that the first fermentor in the multistage system was the only fermentor capable of supporting the growth of L. paracasei under the described conditions. Journal of Industrial Microbiology & Biotechnology (2001) 27, 39–45. Received 26 February 2001/ Accepted in revised form 29 May 2001     

Gastrointestinal survival and potential bioactivities of Lactobacillus curieae CCTCC M2011381 in the fermentation of plant food

Lactobacillus curieae CCTCC M2011381 is a novel strain isolated from stinky tofu brine. In order to evaluate the potential of the strain in fermenting plant foodstuff and possible bioactivity produced in fermentation, the growth of the strain in soymilk, soy protein isolate and ginkgo nut beverages was studied firstly. Cell counts of the strain could increase 4 log CFU/mL after 20 h of growth in all materials. The scavenging ratio of diphenyl picryl hydrazinyl radical was improved by the fermentation due to total flavonoids content increase, peptides formation, and efficient transformation of soy isoflavone glycoside to aglycone. Meanwhile, the fermentation significantly enhanced the inhibitions of 3-hydroxy-3-methylglutaryl-coenzyme A reductase of all the three materials. The highest inhibition of 67.2% was achieved by the fermented ginkgo nut beverage. The fermentation also significantly raise the inhibition of angiotensin I converting enzyme of soy protein isolate and ginkgo nut beverages. Finally, the strain was verified survival in mice gastrointestinal tract within all the fermented matrices. The highest cell count, 5.14 log CFU/g faeces, was detected at 5 d after the gavage of fermented soy protein isolate beverage. Accordingly, L. curieae has the prospect as the starter culture in fermentation of plant foodstuff.     

Development of Probiotic Tablets Using Microparticles: Viability Studies and Stability Studies

Alternative vectors to deliver viable cells of probiotics, to those conferring limited resistance to gastrointestinal conditions, still need to be sought. Therefore the main goal of the study was to develop tablets able to protect entrapped probiotic bacteria from gastric acidity, thus providing an easily manufacturing scale-up dosage form to deliver probiotics to the vicinity of the human colon. Whey protein concentrate microparticles with Lactobacillus paracasei L26 were produced by spray-drying and incorporated in tablets with cellulose acetate phthalate and sodium croscarmellose. The viability of L. paracasei L.26 throughout tableting as well as its gastric resistance and release from the tablets were evaluated. Storage stability of L. paracasei L26 tablets was also performed by evaluation of viable cells throughout 60 days at 23°C and 33% relative humidity. A decrease of approximately one logarithmic cycle was observed after the acid stage and the release of L. paracasei L26 from the tablets occurred only after 4 h in the conditions tested. Microencapsulated L. paracasei L26 in tablets revealed some susceptibility to the storage conditions tested since the number of viable cells decreased 2 log cycles after 60 days of storage. However, the viability of L. paracasei L26 after 45 days of storage did not reveal significant susceptibility upon exposure to simulated gastrointestinal conditions. The developed probiotic tablets revealed to be potential vectors for delivering viable cells of L. paracasei L26 and probably other probiotics to persons/patients who might benefit from probiotic therapy.     

Genomic analysis of Pediococcus starter cultures used to control Listeria monocytogenes in turkey summer sausage.

The pulsed-field technique of clamped homogeneous electric field electrophoresis was employed to characterize and size genomic DNA of three pediocin-producing (Ped+) and two non-pediocin-producing (Ped-) strains of Pediococcus acidilactici. Comparison of genomic fingerprints obtained by digestion with the low-frequency-cleavage endonuclease AscI revealed identical restriction profiles for four of the five strains analyzed. Summation of results for 10 individually sized AscI fragments estimated the genome length to be 1,861 kb for the four strains (H, PAC1.0, PO2, and JBL1350) with identical fingerprints. Genomic analysis of the pediocin-sensitive, plasmid-free strain P. acidilactici LB42 with the unique fingerprint revealed nine AscI fragments and a genome length of about 2,133 kb. Ped- (JBL1350) and Ped+ (JBL1095) starter cultures (one each) were used to separately prepare turkey summer sausage coinoculated with a four-strain Listeria monocytogenes mixture (ca. 10(5) CFU/g). The starter cultures produced equivalent amounts of acid during fermentation, but counts of L. monocytogenes were reduced to a greater extent in the presence of the Ped+ starter culture (3.4 log10 unit decrease) than in the presence of the Ped- starter culture (0.9 log10 unit decrease). Although no listeriae were recovered from sausages following the cook/shower, appreciable pediocin activity was recovered from sausages prepared with the Ped+ strain for at least 60 days during storage at 4 degrees C. The results of this study revealed genomic similarities among pediococcal starter cultures and established that pediocins produced during fermentation provide an additional measure of safety against listerial proliferation in turkey summer sausage.     

Degradation of milk‐based bioactive peptides by yogurt fermentation bacteria*

Aims: To analyse the effect of cell‐associated peptidases in yogurt starter culture strains Lactobacillus delbrueckii ssp. bulgaricus (LB) and Streptococcus thermophilus (ST) on milk‐protein‐based antimicrobial and hypotensive peptides in order to determine their survival in yogurt‐type dairy foods. Methods and Results: The 11mer antimicrobial and 12mer hypotensive milk‐protein‐derived peptides were incubated with mid‐log cells of LB and ST, which are required for yogurt production. Incubations were performed at pH 4·5 and 7·0, and samples removed at various time points were analysed by reversed‐phase high‐performance liquid chromatography (RP‐HPLC). The peptides remained mostly intact at pH 4·5 in the presence of ST strains and moderately digested by exposure to LB cells. Peptide loss occurred more rapidly and was more extensive after incubation at pH 7·0. Conclusions: The 11mer and 12mer bioactive peptides may be added at the end of the yogurt‐making process when the pH level has dropped to 4·5, limiting the overall extent of proteolysis. Significance and Impact of the Study: The results show the feasibility of using milk‐protein‐based antimicrobial and hypotensive peptides as food supplements to improve the health‐promoting qualities of liquid and semi‐solid dairy foods prepared by the yogurt fermentation process.     

Genomic analysis of Pediococcus starter cultures used to control Listeria monocytogenes in turkey summer sausage.

The pulsed-field technique of clamped homogeneous electric field electrophoresis was employed to characterize and size genomic DNA of three pediocin-producing (Ped+) and two non-pediocin-producing (Ped-) strains of Pediococcus acidilactici. Comparison of genomic fingerprints obtained by digestion with the low-frequency-cleavage endonuclease AscI revealed identical restriction profiles for four of the five strains analyzed. Summation of results for 10 individually sized AscI fragments estimated the genome length to be 1,861 kb for the four strains (H, PAC1.0, PO2, and JBL1350) with identical fingerprints. Genomic analysis of the pediocin-sensitive, plasmid-free strain P. acidilactici LB42 with the unique fingerprint revealed nine AscI fragments and a genome length of about 2,133 kb. Ped- (JBL1350) and Ped+ (JBL1095) starter cultures (one each) were used to separately prepare turkey summer sausage coinoculated with a four-strain Listeria monocytogenes mixture (ca. 10(5) CFU/g). The starter cultures produced equivalent amounts of acid during fermentation, but counts of L. monocytogenes were reduced to a greater extent in the presence of the Ped+ starter culture (3.4 log10 unit decrease) than in the presence of the Ped- starter culture (0.9 log10 unit decrease). Although no listeriae were recovered from sausages following the cook/shower, appreciable pediocin activity was recovered from sausages prepared with the Ped+ strain for at least 60 days during storage at 4 degrees C. The results of this study revealed genomic similarities among pediococcal starter cultures and established that pediocins produced during fermentation provide an additional measure of safety against listerial proliferation in turkey summer sausage.